Area Forecast Discussion
Issued by NWS Wilmington, NC

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FXUS62 KILM 300157

Area Forecast Discussion
National Weather Service Wilmington NC
957 PM EDT Wed Mar 29 2017

Relatively cooler and drier high pressure will build in from
the north tonight thru Thursday then shifting offshore Thursday
night. Showers and thunderstorms are expected late Thu night
thru Friday as milder and more humid air returns ahead of an
approaching cold front. The cold front will sweep across the
area and offshore late Friday night. Some of the thunderstorms
may be strong to severe during Friday. The weekend should be
dry as high pressure takes hold. By early next week, the next low
pressure system and associated cold front will produce
showers and thunderstorms as it crosses the area.


As of 930 PM Wednesday...Will go with partly cloudy skies
thruout for the overnight period due to high level clouds
moving over the low amplitude upper ridge axis that nearly lies
overhead. Some mid-level ac could also affect the SC portions of
the ILM CWA. Will also have to keep an eye on the low stratus
clouds that may expand northward into the ILM SC Counties
overnight. With an active ENE to E wind overnight, radiational
fog should not be an issue. In addition, no POPs are in the
fcst for tonight. Very little if any tweaking needed for
tonights lows, unless, the deeper cloudiness expands northeast
across the ILM NC CWA.

As of 3 PM Wednesday...Surface high pressure extending south
across the eastern Great Lakes and into the Carolinas is coincident
with a deep mid and upper level ridge. All of these ridging
features will traverse the area from west to east overnight.

Precipitable water values in the 0.8 to 0.9 inch range tonight are
in the 60-75th percentile for this time of year, indicating this is
not a particularly dry airmass. Plenty of daytime cumulus that has
developed west of the seabreeze should dissipate this evening,
however enough moisture should remain trapped beneath the subsidence
inversion up around 6500 feet AGL that I don`t think we`ll see cloud-
free skies for any significant length of time. Given the presence of
this moisture and also low-level winds expected to not go calm
tonight due to the distant position of the surface high, I have
bumped forecast lows up by a degree or two to 54-57 over NE South
Carolina, and 50-55 over SE North Carolina.

As the deep layer ridge moves offshore Thursday, winds will veer
more southerly above the surface. Gulf moisture arriving in the 850-
700 mb layer during the afternoon could become thick enough to yield
a few showers back in the South Carolina Pee Dee region before
sunset. This should not be a substantial precipitation event during
the day however -- that will hold off until later Thursday night
into Friday. Thursday`s highs should range from 70-74 inland, with
mid to upper 60s within 5-10 miles of the coast.


As of 3 PM Wednesday...Southern stream shortwave moving across the
Mississippi Valley Thu night into Fri will lift a warm front
across the area early in the period. Front is likely to be
accompanied by showers and possibly an isolated thunderstorm but
strong storms seem less likely given the evening timing and
weak dynamics. Best chance for widespread rainfall still appears
to be between 09Z and 15Z Fri, prior to the cold front moving
into the area. Although the timing is less than ideal there will
be an abundance of deep moisture, precipitable water values
rise from around 1" Thu evening to 1.5" Fri morning, and plenty
of forcing. Height falls and low level convergence ahead of the
front should help produce deeper convection, which will have a
40-50 kt low level jet to tap into. The biggest limiting factor
will be the lack of surface based instability given the early
morning timing. Cannot rule out strong or severe storms but at
this point the potential is on the low side, hence SPC`s
marginal risk. Do think heavy rain is almost guaranteed for a
couple hours Fri morning and will continue with inherited
categorical pop. Southerly flow in the wake of the warm front
Thu night and the first part of Fri will keep temps above climo.
Lows will be in the lower 60s Fri morning with highs around 80
Fri afternoon.

Cold front pushes across the area during Fri with guidance
showing a rather impressive dry slot wrapping around the system
aloft. Low level moisture will linger into early afternoon and
may support some light rain/showers, but the significant precip
should end by noon. Although the cold front passes later Fri
there is not much in the way of cold air behind the front and
lows Fri night will still end up about 10 degrees above climo.


As of 300 AM Wednesday...In the wake of this system, dry weather
and above normal temps are expected for the weekend as mid-
level ridging builds across the area and surface high pressure
to our N ridges S. Attention then will turn westward as next
potent southern stream system along the Gulf Coast Sun night
lifts to the NE and drags a warm front to the N. This will again
bring deep moisture into the Carolinas, and with that, showers
and thunderstorms early next week. Timing differences have
decreased slightly and it looks like the greatest risk for
showers and thunderstorms will be Mon night. Showers and
thunderstorms should be increasing from the SW and W Mon
afternoon with the risk decreasing from W to E during the day
Tue as broad low pressure consolidates offshore of the Mid-
Atlantic states Tue and Tue night. Drier air will work into the
eastern Carolinas during Wed.


As of 00Z...VFR conditions thru the 00Z valid 24 hr TAF
Issuance period. However, 1 of the favorite aviation models
indicates a possible IFR ceiling below 1k ft during the
pre-dawn Thu hours across the Myrtle Beach sites. Until
additional models indicate this possibility or model run to run
consistency, will just indicate a FEW/SCT007 for just the MYR
Terminal at this time. Ridging will extend down the east coast
of the U.S. resulting in a somewhat tightened sfc pg across the
area. As a result, looking at NE to ENE winds at 5 to 10 kt
tonight, hiest across the coastal terminals. The sfc pg will
tighten-some Thu aftn veering the winds to the ENE to ESE at 10
to 15 kt. From Thu midday thru Thu evening, will see VFR
ceilings initially that slowly drop to MVFR thresholds by or
during early Thu evening.

Extended outlook...Areas to widespread MVFR/IFR conditions
later Thu night thru Fri afternoon due to showers and strong to
severe thunderstorms. Possible MVFR showers Fri night. VFR
conditions Sat thru early Mon. Possible MVFR/IFR conditions
late Mon due to the approach and passage of a low pressure


As of 930 PM Wednesday...The main high pressure center id
located over southeast Canada tonight. The ridging from it`s
center extends down the U.S. East Coast, with the ridge axis
nearly overhead. The result is not a true NE wind, rather than a
good Easterly component within that wind field. Overall, ENE to
E winds at 10 to 20 kt due to a somewhat tightened sfc pg.
Significant seas will run 3 to 5 ft ILM NC Waters and 2 to 4 ft
for the ILM SC Waters. A slow decaying southeast ground swell
at 9 to 10 second periods continues to help keep overall seas
somewhat elevated. The latest spectral density charts from
NDBC still indicate plenty of power associated with the swell
when compared to the 4 to 6 second wind driven waves.

As of 3 PM Wednesday...A ridge of high pressure extends
southward across the Great Lakes and into the eastern Carolinas.
This ridge should move eastward and off the coast late tonight
and early Thursday morning. With the center of the high so
distant from the Carolinas we will not have a period of light
winds. In fact, wind speeds will increase to a hefty "moderate"
this evening: 15-20 kt with seas building to 4-5 feet away from
the coast. A `Small Craft should exercise caution` headline will
be appended to the forecast through midnight north of Cape

Thursday as the ridge moves farther off the East Coast wind
directions should veer more southeasterly with time with speeds
averaging 15 kt or less.

As of 3 PM Wednesday...Onshore flow Thu night will become
southerly very early Fri as a warm front lifts north of the
waters. Gradient starts to tighten behind the front with
southwest flow increasing to around 20 kt by Fri morning.
Southwest flow remains a solid 20 kt Fri before passage of cold
front brings a slight reduction in the gradient, dropping
offshore flow to 10 to 15 kt late Fri night. Seas build Thu
night in response to increasing southwest flow. At the start of
the period seas will run 2 to 4 ft but quickly build to 4 to 6
ft overnight, peaking in the 4 to 7 ft range later Fri. Offshore
flow behind the exiting cold front will drop seas below 6 ft
around midnight. Anticipate at least a 12 to 18 hour stretch of
SCA conditions during the period.

As of 300 AM Wednesday...Small Craft Advisory conditions
possible late on Mon and Mon night.

Low pressure will be intensifying south of New England Sat
morning. Its attending cold front will be offshore by the start
of this forecast period. Low pressure will continue to intensify
as it moves E and then NE during the weekend. High pressure
centered across eastern Canada will build south across the
Carolina waters during the weekend with the ridge axis moving
offshore Sun night. Next in series of potent southern stream
systems will be approaching from the W on Mon. Its attending
warm front is expected to move across the waters Mon night.

NW winds will generally dominate Sat although an inverted
trough poking N may cause winds to back, especially across
southern waters ahead of reinforcing cold and dry push. Winds
late Sat night and Sun in the wake of this push will be NE. The
wind direction will veer to E later Sun afternoon and then ESE
Sun night. The wind direction will become SE during Mon.

The highest wind speeds during this period are expected on Mon
when they are expected to increase to 20 to 25 kt late day and
Mon night. Wind speeds Sat through Sun night should be mainly 5
to 15 kt.

Seas will be 3 to 4 ft Sat morning, subsiding to mainly 2 to 3
ft Sat night and then 1 to 2 ft Sun. There will be a tendency
for backswell to slow the slow the rate of subsiding seas
through the first part of the weekend. Expect seas will reverse
higher late Sun night and Mon, possibly reaching Small Craft
Advisory levels Mon night.




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